Earlier onset of puberty in girls: Relation to increased body mass index and race

Objective. A recent study conducted by the Pediatric Research in Office Settings network provided evidence that girls in the United States, especially black girls, are starting puberty at a younger age than earlier studies had found, but the reasons for this are not known. Because nutritional status is known to affect timing of puberty and there is a clear trend for increasing obesity in US children during the past 25 years, it was hypothesized that the earlier onset of puberty could be attributable to the increasing prevalence of obesity in young girls. Therefore, the objective of this study was to reexamine the Pediatric Research in Office Settings puberty data by comparing the age-normalized body mass index (BMI-ZS; a crude estimate of fatness) of girls who had breast or pubic hair development versus those who were still prepubertal, looking at the effects of age and race. Results. For white girls, the BMI-ZS were markedly higher in pubertal versus prepubertal 6- to 9-year-olds; for black girls, a smaller difference was seen, which was significant only for 9-year-olds. Higher BMI-ZS also were found in girls who had pubic hair but no breast development versus girls who had neither pubic hair nor breast development. A multivariate analysis confirms that obesity (as measured by BMI) is significantly associated with early puberty in white girls and is associated with early puberty in black girls as well, but to a lesser extent. Conclusions. The results are consistent with obesity's being an important contributing factor to the earlier onset of puberty in girls. Factors other than obesity, however, perhaps genetic and/or environmental ones, are needed to explain the higher prevalence of early puberty in black versus white girls.